Presently manufactured fuel lines for automobiles are made of relatively thick walled tubes of nylon 11 of 12. These materials provide the desired resistance to fuels such as gasoline, both leaded, unleaded and supra, diesel fuel, methanol, ethanol and sour gas. The nominal wall thickness of such tubes is approximately forty-thousandths of an inch (1 mm), since this provides the desired mechanical requirements of burst pressure (500 psig, minimum), sag resistance, kink resistance, cold impact resistance (at least 1 ft. lb. at minus 40.degree. C.) and general handling durability. For fuel transport alone, however, a wall thickness of only one- to two-thousandths of an inch is actually necessary. The remainder of the wall thickness is necessary to protect and support the inner, fuel containing thickness, as well as to provide the necessary mechanical properties to the tube.
In order to properly route the fuel line throughout the vehicle, turns and bends are generally thermally preformed into the thick tube.
In the same manner, vapor tubes for recycling the fuel vapors in an automobile pollution control device must be resistant to the fuel as well as to combustion vapors. For this use, the tubing must be made of a fuel and combustion gas resistant material which, as with fuel lines, are generally thermally preformed.
One significant problem with the use of thermoplastic tubes (such as polyethylene, polypropylene, polyvinyl chloride or the like) is the relatively high gas vapor permeation of those materials. Therefore, in order to successfully transport fuel, tubes of those thermoplastic material must be surface treated to block or reduce such vapor transmission. Furthermore, no suitable method currently exists for surface coating the inner diameter of extruded thermoplastic tubing.
Also, for many applications, plastic tubing cannot prudently be used because of the work environment. Plastic tubing in the area of a welding operation is subject to rupture by weld splatter which may melt the tube wall. This occurrence could have disastrous effects if the tube is carrying a flammable substance, such as the fuel. Alternatively, to prevent against excessive abrasion or hot spots in localized areas, it is possible to provide this plastic tube with additional components, such as protective sleeves of rubber, metal or the like, in such localized areas. A disadvantage of this solution is that it is not always known where such problem areas will be encountered.
Therefore, thin walled metal tubing has been considered for this application, but it has not been found satisfactory due to its poor bending characteristics. A thin tube wall on an inner bend radius kinks very readily due to its inability to withstand a compressive load.
Flexible hose assemblies of plastic and metal for fuel lines are shown in the prior art. U.S. Pat. No. 2,787,289 to Press discloses a flexible line made of an extruded polytetrafluoroethylene tube which is surrounded by one or more layers of reinforcing wire braid of stainless steel and covered by a flexible tubular cover which is made of asbestos impregnated with a substantially oil proof material. This type of line, although flexible, is also resilient so that it is not capable of maintaining the shape into which it is bent. This is undesirable for many automotive applications since at least some type of semi-rigidity or shape holding property is necessary to maintain the line in the proper position.
In U.S. Pat. No. 4,327,248 to Campbell, there is disclosed a shielded electrical cable. The shield is made of a flexible metal tape with a coating of a copolymer of ethylene with a monomer having a reactive carboxyl group bonded to at least one of its sides. An adhesive is used to bond the coating to a flexible or semi-rigid non-olefinic polymeric material.
The present invention resolves the deficiencies of the prior art and provides a low cost plastic to metal bonded encasement which can be mechanically formed or bent to a predetermined configuration and which further can retain its shape after being formed or bent without kinking. Furthermore, this composite tube provides increased resistance to abrasion, hot spots and vapor permeation than those of the prior art, due to its ability to rapidly dissipate heat axially and radially along the composite tube.